Electrosurgical jaw structure for controlled energy delivery

ABSTRACT

A working end of a surgical instrument that carries first and second jaws for delivering energy to tissue. In a preferred embodiment, at least one jaw of the working end defines a tissue-engagement plane that contacts the targeted tissue. The cross-section of the engagement plane reveals that it defines a surface conductive portion that overlies a variably resistive matrix of a temperature-sensitive resistive material or a pressure-sensitive resistive material. An interior of the jaw carries a conductive material or electrode that is coupled to an Rf source and controller. In an exemplary embodiment, the variably resistive matrix can comprise a positive temperature coefficient (PTC) material, such as a ceramic, that is engineered to exhibit a dramatically increasing resistance (i.e., several orders of magnitude) above a specific temperature of the material.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/032,867, filed Oct. 22, 2001, and issued as U.S. Pat. No. 6,929,644,titled Electrosurgical Jaw Structure for Controlled Energy Delivery, thefull disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrosurgical jaws and methods fordelivering energy to tissue, and more particularly to an instrumentworking end for grasping tissue that self-modulates energy applicationto engaged tissues for sealing, welding or coagulating purposes.

2. Description of the Related Art

In various open and laparoscopic surgeries, it is necessary tocoagulate, seal or weld tissues. One preferred means of tissue-sealingrelies upon the application of electrical energy to captured tissue tocause thermal effects therein for sealing purposes. Various mono-polarand bi-polar radiofrequency (Rf) jaw structures have been developed forsuch purposes. In a typical bi-polar jaw arrangement, each jaw facecomprises an electrode and Rf current flows across the captured tissuebetween the first and second polarity electrodes in the opposing jaws.While such bi-polar jaws can adequately seal or weld tissue volumes thathave a small cross-section, such bi-polar instruments often areineffective in sealing or welding many types of tissues, such asanatomic structures having walls with irregular or thick fibrouscontent, bundles of disparate anatomic structures, substantially thickanatomic structures, or tissues with thick fascia layers such as largediameter blood vessels.

Prior art Rf jaws that engage opposing sides of a tissue volumetypically cannot cause uniform thermal effects in the tissue, whetherthe captured tissue is thin or substantially thick. As Rf energy densityin tissue increases, the tissue surface becomes desiccated and resistantto additional ohmic heating. Localized tissue desiccation and charringcan occur almost instantly as tissue impedance rises, which then canresult in a non-uniform seal in the tissue. The typical prior art Rfjaws can cause a further undesirable effects by propagating Rf densitylaterally from the engaged tissue to cause unwanted collateral thermaldamage.

What is needed is an instrument with a jaw structure that can apply Rfenergy to tissue in new modalities: (i) to weld or seal tissue volumesthat have substantial fascia layers or tissues that are non-uniform inhydration, density and collagenous content; (ii) to weld a targetedtissue region while substantially preventing thermal damage in regionslateral to the targeted tissue; and (iii) to weld a bundle of disparateanatomic structures.

BRIEF SUMMARY OF THE INVENTION

The principal objective of the present invention is to provide aninstrument and jaw structure that is capable of controllably applyingenergy to engaged tissue. As background, the biological mechanismsunderlying tissue fusion by means of thermal effects are not fullyunderstood. In general, the application of Rf energy to a capturedtissue volume causes ohmic heating (alternatively described as active Rfheating herein) of the tissue to thereby at least partially denatureproteins in the tissue. By ohmic heating, it is meant that the active Rfcurrent flow within tissue between electrodes causes frictional orresistive heating of conductive compositions (e.g., water) in thetissue.

One objective of the invention is to denature tissue proteins, includingcollagen, into a proteinaceous amalgam that intermixes and fusestogether as the proteins renature. As the treated region heals overtime, the so-called weld is reabsorbed by the body's wound healingprocess. A more particular objective of the invention is to provide asystem that (i) instantly and automatically modulates ohmic heating oftissue to maintain a selected temperature in the tissue, and (ii) toinstantly and automatically modulate total energy application betweenactive Rf heating (resulting from tissue's resistance to current flowtherethrough) and conductive heating of tissue that results from heatconduction and radiation from resistively heated jaw components.

In general, the various jaw structures corresponding to the presentinvention all provide an Rf working end that is adapted to instantly andautomatically modulate between active Rf heating of tissue andconductive heating of tissue by resistive jaw portions. Thus, thetargeted tissue can be maintained at a selected temperature for aselected time interval without reliance of prior art “feedback”monitoring systems that measure impedance, temperature, voltage or acombination thereof.

In an exemplary embodiment, at least one jaw of the instrument defines atissue-engagement plane that engages the targeted tissue. Across-section of the jaw inwardly of the engagement plane illustratesthat multiple electrically-conductive components comprise the inventionfor applying energy to tissue. Typically, the engagement plane defines asurface conductive portion (for tissue contact) that overlies a medialportion of a variably resistive material. An exemplary jaw furthercarries a core conductive material (or electrode) that is coupled to anRf source and controller. Of particular interest, one embodiment has avariably resistive matrix that comprises a positive temperaturecoefficient (PTC) material having a resistance (i.e., impedance toelectrical conduction therethrough) that changes as it increases intemperature. One type of PTC material is a ceramic that is engineered toexhibit a dramatically increasing resistance above a specifictemperature of the material, sometimes referred to as a Curie point or aswitching range.

In one embodiment, a jaw of the working end utilizes a medial variablyresistive matrix that has a selected switching range, for example a5°–20° C. range, which approximates a targeted temperature that issuitable for tissue welding. In operation, it can be understood that theengagement plane will apply active Rf energy to (or cause ohmic heatingwithin) the engaged tissue until the point in time that the variablyresistive matrix is heated to its selected switching range. When thetissue temperature thus elevates the temperature of the PTC material tothe switching range, Rf current flow from the core conductive electrodethrough to the engagement surface will be terminated due to thetemperature increase in tissue and the resistive matrix. This instantand automatic reduction of Rf energy application can be relied on toprevent any substantial dehydration of tissue proximate to the probe'sengagement plane. By thus maintaining an optimal level of moisturearound the engagement plane, the working end can more effectively applyenergy to the tissue—and provide a weld thicker tissues with limitedcollateral thermal effects.

The working end of the probe corresponding to the invention furtherprovides a suitable cross-section and mass for providing a substantialheat capacity. Thus, when the medial variably resistive matrix iselevated in temperature to its switching range, the matrix caneffectively function as a resistive electrode to thereafter passivelyconduct thermal energy to the engaged tissue volume. Thus, in operation,the working end can automatically modulate the application of energy totissue between active Rf heating and passive conductive heating of thetargeted tissue to maintain the targeted temperature level.

In another preferred embodiment of the invention, the variably resistivematrix can be a silicone-based material that is flexible andcompressible. Thus, the engagement surface of one or both jaws canflexibly engage tissue to maintain tissue contact as the tissue shrinksduring the welding process. In a related embodiment, the variablyresistive matrix can be an open-cell silicone-based material that iscoupled to a fluid inflow source for delivering fluid to the engagementplane to facilitate welding of very thin tissue volumes.

The jaws of the invention can operate in mono-polar or bi-polarmodalities, with the variably resistive matrix carried in either or bothjaws of the working end.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will be understoodby reference to the following detailed description of the invention whenconsidered in combination with the accompanying Figures, in which likereference numerals are used to identify like elements throughout thisdisclosure.

FIG. 1 is perspective view of a Type “A” working end of the inventionshowing first and second jaws carried at the end of an introducer.

FIG. 2 is a partial sectional view of a portion of the jaws of FIG. 1taken along line 2—2 of FIG. 1 showing the active electrical energydelivery components corresponding to the invention.

FIG. 3 is a graph of the temperature vs. resistance profile of thepositive temperature coefficient (PTC) matrix of the jaws of FIGS. 1–2.

FIG. 4 is a graph showing the temperature-resistance profile of the PTCmatrix, the impedance of tissue and the combined resistance of the PTCmatrix and tissue that is readable by feedback circuitry.

FIG. 5 is a sectional view of a Type “B” jaw structure that carriesexposed first and second polarity electrodes and a variably resistive orPTC matrix in an engagement surface of a first jaw with an insulatedsecond jaw, the first and second polarity electrodes adapted, in part,for bi-polar Rf energy application to engaged tissue.

FIG. 6 is a sectional view of an alternative Type “B” jaw structuresimilar to FIG. 5 that carries cooperating first and second polarityelectrodes in both jaws, and a variably resistive matrix in a singlejaw.

FIGS. 7A–7B are sectional views of another alternative Type “B” jawstructure similar to FIG. 6 that carry cooperating first and secondpolarity electrodes in both jaws, together with a variably resistivematrix in both jaws.

FIG. 8 is a sectional view of another Type “B” jaw structure similar toFIG. 6 that carry cooperating first and second polarity electrodes inboth jaws, together with a variably resistive matrix in both jaws.

FIG. 9 is a sectional view of another Type “B” jaw structure similar toFIG. 8 that carries cooperating first and second polarity electrodes ineach jaws, together with a variably resistive matrix in each jaw and aninsulative outer layer.

FIG. 10 is a sectional view of another Type “B” jaw structure similar toFIG. 8 that has a first engagement plane with an exposed first polarityelectrode and an interior variably resistive matrix and a secondengagement plane with an exposed variably resistive matrix and aninterior second polarity electrode, each jaw having an insulative outerlayer.

FIG. 11 is a sectional view of yet another Type “B” jaw structure thathas a first engagement plane with an exposed variably resistive matrixand an interior first polarity electrode, a second engagement plane withan exposed second polarity electrode.

FIG. 12 is a sectional view of another Type “B” jaw structure that has afirst and second engagement planes with an exposed variably resistivematrix and interior first and second polarity electrode, respectively.

FIG. 13 is a perspective view of a Type “C” working end in accordancewith the invention that has a jaw engagement plane with a variablyresistive (PTC) material that is flexible or compressible.

FIG. 14 is a sectional view of the jaws of FIG. 13 showing the variablyresistive (PTC) material and exposed electrode surfaces.

FIG. 15 is a sectional view of a Type “D” jaw in accordance with theinvention that has a jaw engagement plane comprising a surface of anopen-cell, compressible variably resistive (PTC) material together witha fluid source is coupled thereto for delivering a fluid to theengagement plane.

FIG. 16 is a cut-away view of a portion of a Type “E” jaw that carriesopposing polarity electrodes with spaced apart volumes of athermally-sensitive resistive matrix.

FIG. 17 is a sectional view of a portion of a Type “F” jaw structurethat carries variably resistive layer that is pressure sensitive.

DETAILED DESCRIPTION OF THE INVENTION

1. Type “A” Working End for Tissue Sealing

An exemplary Type “A” working end 100 of a surgical grasping instrumentis illustrated in FIGS. 1–2 that is adapted for energy delivery forsealing or welding tissue. The working end 100 is carried at the distalend of an introducer portion 102 that can be rigid or flexible in anysuitable diameter. For example, the introducer portion 102 can have adiameter ranging from about 3 mm. to 5 mm. (or larger) for use inendoscopic surgical procedures. The introducer portion extends alongaxis 107 from its proximal end that is connected to a handle (notshown). The working end has first (lower) jaw 105A and second (upper)jaw 105B that are coupled to the distal end 108 of the introducerportion 102. The jaws may both be moveable or a single jaw may move toprovide an open position and a closed position wherein the jawsapproximate toward axis 107. The opening-closing mechanism can be anytype known in the art. For example, a reciprocatable cam-type member 109can slide over the jaws 105A and 105B to engage the outer surfaces ofthe jaws, and can be the type of mechanism disclosed in co-pending U.S.patent application Ser. No. 09/792,825 filed Feb. 24, 2001 titledElectrosurgical Working End for Transecting and Sealing Tissue, which isincorporated herein by reference.

In the exemplary embodiment of FIG. 2, the first (lower) jaw 105A has atissue-engaging surface or engagement plane 125 that contacts anddelivers energy to a targeted tissue. The jaws can have any suitablelength with teeth or serrations in any location for gripping tissue, andis shown in FIG. 2 with such serrations 127 along the outboard portionsof the jaws thus leaving the engagement plane 125 inward of theserrations. In the embodiments described below, the engagement plane 125generally is shown with a non-serrated surface for clarity ofexplanation, but the engagement plane 125 itself can be any non-smoothgripping surface.

In the exemplary embodiment of FIG. 2, the engagement surface or plane125 that delivers energy to tissue extends along an axial length of onlythe lower jaw 105A, and the tissue-contacting surface 130 of the secondjaw 105B is passive and comprises an insulated material 131 or has aninsulated surface layer. As will be described below, alternativeembodiments corresponding to the invention provide both the first andsecond jaws with electrically “active” components.

The sectional view of FIG. 2 more particularly illustrates theindividual electrically relevant components within the body of the lowerjaw for controllably delivering energy to tissue for sealing or weldingpurposes. The engagement surface 125 of jaw 105A has a conductivesurface material indicated at 135 that is both electrically conductiveand thermally conductive. For example, the conductive surface layer 135can be a thin film deposit of any suitable material known in the art(e.g., gold, platinum, palladium, silver, stainless steel, etc.) havingany suitable thickness dimension d₁, for example, ranging from about of0.0001″ to 0.020″. Alternatively, the conductive layer 135 can comprisea machined or cast metal having a more substantial thickness that isconductively bonded to the interior layers described next.

As can be seen in FIG. 2, the jaw 105A has a medial (second) material ormatrix 140 that is variably resistive (alternatively called variablyconductive herein) and carried inwardly of the surface conductivematerial 135. Further, the body of jaw 105A carries a (third) interiorconductive material or electrode 145 at its core. The medial conductivelayer 140 thus is intermediate the engagement plane 125 and the interiorconductive material 145. The third conductive material or electrode 145is coupled by an electrical lead to a remote voltage (Rf) source 150 andoptional controller 155. The medial variably resistive matrix 140 canhave any suitable cross-sectional dimensions, indicated generally at d₂and d₃, and preferably such a cross-section comprises a significantfractional volume of the jaw body to provide a thermal mass foroptimizing passive conduction of heat to tissue as will be describedbelow.

It can be easily understood from FIG. 2 that the core conductivematerial 145 is coupled to, or immediately adjacent to, the medialvariably resistive material 140 for conducting electrical energy fromthe interior conductor to engaged tissue through matrix 140. In FIG. 2,it can be seen that the first, second and third components (indicated at135, 140, 145) are carried in a structural body component 148 of the jaw105A that can be any suitable metal with an insulative coating or anyother rigid body that can accommodate loads on the jaw as it engages andcompresses tissue.

Of particular interest, still referring to FIG. 2, the medial variableconductive matrix 140 comprises a polymeric material having atemperature-dependent resistance. Such materials are typically known inthe art as polymer-based temperature coefficient materials, andsometimes specifically described as thermally-sensitive resistors orthermistors that exhibit very large changes in resistance with a smallchange of body temperature. This change of resistance with a change intemperature can result in a positive coefficient of resistance where theresistance increases with an increase in temperature (PTC or positivetemperature coefficient material). The scope of the invention alsoincludes a medial variably conductive matrix 140 of a negativetemperature coefficient (NTC) material wherein its resistance decreaseswith an increase in temperature.

In one preferred embodiment, the PTC matrix 140 is a ceramic layer thatcan be engineered to exhibit unique resistance vs. temperaturecharacteristics and can maintain a very low base resistance over a widetemperature range, with a dramatically increasing resistance above aspecific temperature of the material (sometimes referred to as a Curiepoint or switching range; see FIG. 3). One aspect of the inventionrelates to fabrication of the medial PTC matrix 140 to have a selectedswitching range between a first temperature (T₁) and a secondtemperature (T₂) that approximates the targeted tissue temperature inthe contemplated tissue sealing or welding objective. The selectedswitching range, for example, can be any substantially narrow 1°–10° C.range that is determined to be optimal for tissue sealing or welding(e.g., any 5° C. range between about 65°–200° C.). A more preferredswitching range can fall within the larger range of about 80°–100° C.

In operation, it can be understood that the delivery of Rf energy to theinterior conductor 145 will be conducted through the variably conductivematrix 140 and the engagement plane 125 to thereby apply Rf energy (oractive ohmic heating) to tissue engaged between the jaws 105A and 105B(see FIG. 2). After the engaged tissue is elevated in temperature bysuch active Rf heating, the lower jaw's conductive surface layer 135 andthe medial conductive layer 140 will be elevated to the selectedswitching range. Thereafter, the mass of the body will be modulated intemperature, similar to the engaged tissue, at or about the selectedswitching range. Thereafter, the jaws body will conduct or radiatethermal effects to the engaged tissue.

In other words, the critical increase in temperature of the variablyresistive matrix 140 is typically caused by the transient hightemperature of tissue that is caused by active Rf heating of the tissue.In turn, heat is conducted back through the layer of the firstconductive material 135 to medial matrix 140. A suitable variablyresistive PTC material can be fabricated from high puritysemi-conducting ceramics, for example, based on complex titanatechemical compositions (e.g., BaTiO₃, SrTiO₃, etc.). The specificresistance-temperature characteristics of the material can be designedby the addition of dopants and/or unique materials processing, such ashigh pressure forming techniques and precision sintering. Suitablevariably resistive or PTC materials are manufactured by a number ofsources, and can be obtained, for example from Western ElectronicComponents Corp., 1250-A Avenida Acaso, Camarillo, Calif. 93012. Anothermanner of fabricating the medial conductive material 140 is to use acommercially available epoxy that is doped with a type of carbon. Infabricating a substantially thin medial conductive layer 140 in thismanner, it is preferable to use a carbon type that has single molecularbonds. It is less preferable to use a carbon type with double bondswhich has the potential of breaking down when used in thin layers, thuscreating the potential of an electrical short circuit between conductiveportions 145 and 135.

As can be seen in FIG. 2, the core conductive material or electrode 145is operatively connected to the voltage (Rf) source 150 by a firstelectrical lead 156 a that defines a first polarity of the Rf source. Inthis preferred embodiment, the conductive engagement surface 135 iscoupled to a second electrical lead 156 b that defines a second oropposing polarity of the Rf source 150. A ground pad indicated at 158 inFIG. 2 first lead 156 a to accomplish a preferred method of theinvention, as will be described below.

The manner of utilizing the working end 100 of FIG. 2 to perform amethod of the invention can be understood as engaging and compressingtissue between the first and second jaws 105A and 105B and thereafterapplying active Rf energy to the tissue to maintain a selectedtemperature for a selected time interval. For example, the instrument isprovided with a working end that carries a medial variably conductormatrix 140 (see FIG. 2) that has a switching range at or about 90° C. atwhich its resistance increases greater than about 5% (and can be as muchas 1,000,000% or more) above its low base resistively with a change intemperature of about 5° C. or less (see FIG. 3).

With the jaws in the closed position and the engagement plane 125engaging tissue, the operator actuates a switch 164 that delivers Rfenergy from the voltage (Rf) source 150 to the interior conductor 145.At ambient tissue temperature, the low base resistance of the medialconductive matrix 140 allows unimpeded Rf current flow from the voltagesource 150 through the engagement surface 125 (and conductor layer 135)and tissue to return electrical lead 156 a that is coupled to ground pad158. It can be understood that the engaged tissue initially will have asubstantially uniform impedance to electrical current flow, which willincrease substantially in proximity to engagement surface 125 as theengaged tissue loses moisture due to the active Rf delivery.

Following an arbitrary time interval, the impedance of tissue proximateto engagement surface 125 typically will be elevated, and the highertissue temperature will instantly conduct heat to the medial PTC matrix140. In turn, the medial PTC layer 140 will reach its switching rangeand terminate Rf current flow from the core conductor 145 to theengagement surface 125. Such automatic reduction of active Rf energyapplication will prevent any substantial dehydration of tissue proximateto the engagement plane 125. By thus maintaining the desired level ofmoisture in tissue proximate to the engagement plane 125, the workingend can more effectively apply energy to the tissue. Such energyapplication can extend through thick engaged tissue volumes whilecausing very limited collateral thermal effects. Thereafter, as thetemperature of tissue proximate to engagement surface 125 falls bythermal relaxation and the lack of an Rf energy density, the temperatureof the medial conductive matrix 140 will thus fall below the thresholdof the selected switching range. This effect, in turn, will cause Rfcurrent to again flow through the assembly of conductive layers 145, 140and 135 to the engaged tissue to again increase the tissue temperatureby active Rf heating. By the above-described mechanisms of causing themedial variably resistive matrix 140 to hover about its selectedswitching range, the actual Rf energy applied to the engaged tissue canbe precisely modulated to maintain the desired temperature in thetissue.

Of particular interest, in one embodiment, the polymer matrix thatcomprises the medial conductor portion 140B is doped with materials toresistively heat the matrix as Rf energy flow therethrough is reduced.Thus, the thermal mass of the jaws which are elevated in temperature candeliver energy to the engaged tissue by means of greater passiveconductive heating—at the same time Rf energy delivery causes lesseractive tissue heating. This balance of active Rf heating and passiveconductive (or radiative) heating can maintain the targeted temperaturefor any selected time interval.

In summary, one method of the invention comprises the delivery of Rfenergy from a voltage source 150 to a conductive jaw surface 135 througha thermally-sensitive resistor material 140 wherein the resistormaterial has a selected switching range that approximates a targetedtemperature for tissue sealing or welding. In operation, the working endautomatically modulates active Rf energy density in the tissue as thetemperature of the engaged tissue conducts heat back to thethermally-sensitive resistor material 140 to cause its temperature toreach the selected switching range. In this range, the Rf current flowwill be reduced, with the result being that the tissue temperature canbe maintained in the selected range without the need for thermocouplesor any other form of feedback circuitry mechanisms to modulate Rf powerfrom the source. Most important, it is believed that this method of theinvention will allow for immediate modulation of actual Rf energyapplication along the entire length of the jaws, which is to becontrasted with prior art instruments that utilize a temperature sensorand feedback circuitry. Such sensors or thermocouples measuretemperature only at a single location in the jaws, which typically willnot be optimal for energy delivery over the length of the jaws. Suchtemperature sensors also suffer from a time lag. Further, suchtemperature sensors provide only an indirect reading of actual tissuetemperature—since a typical sensor can only measure the temperature ofthe electrode.

Another method of the invention comprises providing the working end witha suitable cross-section of variably resistive matrix 140 so that whenit is elevated in temperature to a selected level, the conductive matrix140 effectively functions as a resistive electrode to passively conductthermal energy to engaged tissue. Thus, in operation, the jaws canautomatically modulate the application of energy to tissue betweenactive Rf heating and passive conductive heating of the targeted tissueat a targeted temperature level.

FIG. 3 illustrates another aspect of the method of the invention thatrelates to the Rf source 150 and controller 155. A typical commerciallyavailable radiofrequency generator has feedback circuitry mechanismsthat control power levels depending on the feedback of impedance levelsof the engaged tissue. FIG. 3 is a graph relating to the probe ofpresent invention that shows: (i) the temperature-resistance profile ofthe targeted tissue, (ii) the temperature-resistance profile of the PTCconductive matrix 140 of the probe, and (iii) the combinedtemperature-resistance profile of engaged tissue and the PTC conductivematrix. In operation, the Rf source 150 and controller 155 can read thecombined impedance of the engaged tissue and the PTC conductive layerwhich will thus allow the use of the instrument with any typical Rfsource without interference with feedback circuitry components.

2. Type “B” Jaw Structures

A series of exemplary Type “B” jaw structures 200 a–200 i correspondingto the invention are illustrated in FIGS. 5–12. Each set of paired firstand second jaws, 205A and 205B, are shown in sectional view toillustrate more specifically how a thermally-dependent variablyresistive layer (indicated at 240 in FIGS. 5–12) can be configured tocooperate with at least one electrode to (i) apply active Rf energy totissue engaged between the jaws while the temperature of the variablyresistive matrix is below its switching range, and (ii) to causemodulation of both active and passive heating when the variablyresistive matrix hovers about its switching range.

FIG. 5 illustrates working end 200 a wherein lower (first) jaw 205Adefines an engagement plane 225A that contacts tissue. In thisembodiment, the upper (second) jaw 205B defines a tissue-contactingplane 225B that is a surface of an insulator material 226. Thestructural components of the jaws indicated at 228 a and 228 b, ifrequired for strength, are of an insulated material or separated fromthe electrical body components by an insulative layer. Now turning tothe active components of the working end 200 a, the thermally-dependentresistive layer 240 is exposed to the engagement plane 225A at regions244 a and 244 a′. In this embodiment of FIG. 5, the thermally-dependentresistive layer 240 is intermediate to the opposing polarity electrodes245A and 245B as defined by the circuitry coupled to voltage source 150(see FIG. 1). For convenience, the electrodes are indicated throughoutthis disclosure as positive (+) and (−) polarities at a particular pointin time. The electrodes 245A and 245B have surface portions 247 a and247 b exposed in the engagement plane of the lower jaw. It should beappreciated that the size and shape of structural body components 228a–228 b can be varied, and may not be required at all. For example, ajaw as depicted in FIG. 5 can use a substantially strong metal forelectrode 245A which can comprise the structural body component of thejaw, with or without a thin insulative coating outside the engagementplane 225A. For clarity of explanation, the gripping elements that aretypically used in the jaw surface are not shown.

It can be understood from FIG. 5 that active heating of the targetedtissue tt (phantom view) will occur generally from current flow betweenfirst polarity electrode 245A and second polarity electrode 245B (seearrows A). Such current flow also can cooperate with a separate butoptional “ground-pad” used as a return electrode (cf. FIG. 2). It can befurther understood that the elevation of the tissue temperature of themedial PTC matrix 240 will then modulate energy application between (i)active Rf heating, and (ii) passive or conductive heating. For thisreason, in this embodiment as the others described next, the medial PTCmatrix 240 preferably comprises a substantial portion of the jaw bodyfor retaining heat for such conductive heating.

FIG. 6 illustrates working end 200 b with the lower (first) jaw 205Adefining 20 engagement plane 225A and the upper (second) jaw 205Bdefining engagement plane 225B. This embodiment is very similar to theembodiment of FIG. 5, except that the upper engagement plane 225Bcomprises a surface of an active conductive body indicated at 245B′ thatis coupled to a voltage source. The electrode 245B′ has a polaritycommon with electrode 245B, or alternatively can have a polarity commonwith electrode 245A (not shown). It can be understood from FIG. 6 thatactive heating of tissue engaged between the jaws will occur as currentflows between second polarity electrodes 245B–245B′ and the firstpolarity electrode 245A. The working can also cooperate with a separate“ground-pad” that functions as a return electrode (not shown). Asdescribed previously, the elevation of the temperature of medial PTCmatrix 240 again will modulate energy application between (i) active Rfheating, and (ii) passive or conductive heating about its selectedswitching range. It should be appreciated that jaws 205A and 205B asdepicted FIG. 6 with opposing polarity electrodes in direct oppositionto one another can be provided with further means for preventing theelectrodes from contacting each other as the jaws are pressed together.The perimeter of the jaws or the jaw's engagement planes can carrysurface elements indicated at 249 (phantom view) that prevent contact ofthe opposing polarity electrodes. For example, the active electrodesurfaces can be slightly recessed relative to such elements 249.

FIGS. 7A–7B illustrate working ends 200 c and 200 d that are similar andshow optional configurations of jaws 205A and 205B that defineengagement planes 225A and 225B, respectively. Each jaw in theembodiments of FIGS. 7A–7B carry opposing polarity electrodes with anintermediate layer of a thermally-dependent resistive matrix 240. InFIG. 7A, electrodes 245A and 245B have exposed surfaces 244 a and 244 bin the lower jaw's engagement plane 225A. Similarly, electrodes 245A′and 245B′ have exposed surfaces 244 a′ and 244 b′ in engagement plane225B of the upper jaw. The electrode arrangement of FIG. 7B differs onlyin the spatial location of the opposing polarity electrode surfaces. Inuse, it can be understood how Rf active heating of engaged tissue willoccur as current flows between first polarity electrodes in each jaw—andbetween the jaws. Again, the elevation of the temperature of medial PTCmatrix 240 will modulate energy application between active Rf heatingand passive heating at the selected switching range.

In another similar electrode arrangement (not shown), a plurality ofcommon polarity electrodes can be exposed in an engagement surface witha phase shift in the voltage delivered to such electrodes as provided bythe voltage source or sources. Such phase shift electrodes can cooperatewith a return electrode in either jaw's engagement surface and/or aground pad. The paired jaw's engagement surfaces also can be configuredwith mirror image electrodes, which can be phase shift electrodes whichcan reduce capacitive coupling among such electrode arrangements. Ingeneral, such phase shift features can be combined with any of theworking ends of FIGS. 6–12.

FIG. 8 illustrates a sectional view of another embodiment of working end200 e with jaws 205A and 205B that define engagement planes 225A and225B, respectively. In this embodiment, each jaw's engagement planecarries at least two spaced apart electrode surfaces with opposingpolarities. For example, lower jaw 205A carries first polarityelectrodes 245A and second polarity electrode 245B (collectively).Between the electrodes 245A and 245B is an intermediatethermally-dependent resistive matrix 240. The upper jaw 205B carriesfirst polarity electrodes 245A′ (collectively) and second polarityelectrode 245B′ with thermally-dependent resistive matrix 240therebetween. Such a jaw configuration will modulate the application ofenergy to tissue as described previously. FIG. 9 illustrates a sectionalview of the jaws of working end 200 f which is similar to FIG. 8 exceptthat the jaws have an outer perimeter of a insulator material indicatedat 251.

FIGS. 10 and 11 illustrate sectional views of other embodiments ofworking end 200 g and 200 h with jaws 205A and 205B that defineengagement planes 225A and 225B, respectively. In these embodiments, onejaw's engagement plane has an active exposed electrode surface indicatedat 245B, while the opposing jaw carries an opposing polarity electrode245A at its interior with a thermally-dependent resistive matrix 240 atthat jaw's engagement surface. The jaw assembly of FIG. 10 further showsan optional insulative layer 251 about the exterior of the jaws. Theresistive matrix comprises a substantial portion of the jaw's mass, andthus is adapted to modulate the application of energy to tissue asdescribed previously.

FIG. 12 illustrates another sectional view of a jaw assembly 2001corresponding to the invention with jaws 205A and 205B that defineengagement planes 225A and 225B, respectively. In this embodiment, eachjaw's engagement plane 225A and 225B comprises a surface of athermally-dependent resistive matrix 240 with neither of the opposingpolarity electrodes 245A and 245B being exposed for tissue contact.

3. Type “C” Jaw Structure for Sealing Tissue

An exemplary Type “C” jaw structure 300 carried by introducer 310corresponding to the invention is illustrated in FIGS. 13–14. The Type“C” system differs in that it utilizes a different form ofthermally-dependent resistive layer (indicated at 340 in FIGS. 13–14)that is an elastomer, for example a silicon-based sponge-type materialthat can be resilient or compressible. More in particular, FIG. 13illustrates working end 300 with lower (first) jaw 305A definingengagement plane 325A that contacts tissue. The upper (second) jaw 305Bdefines a tissue-contacting plane 325B that is a surface of an insulatormaterial 326, but it can also carry electrically conductive componentsas generally depicted in FIGS. 6–12. The jaw structure of FIG. 13further shows that it is configured with a central channel or slot 332that is adapted to accommodate a reciprocatable tissue-cutting member333 for transecting sealed tissue. Such a moveable cutting member 333 isactuated from the instrument handle (not shown) as is known in the art.The cutting member 333 can be a sharp blade or an Rf cutting electrodethat is independently coupled to a high voltage Rf source. Thisembodiment shows tissue-gripping serrations 334 along an inner portionof the jaws, but any location is possible. It should be appreciated thatelectrode components and thermally-dependent resistive components of theinvention can be adapted for any jaws, or left-side and right-side jawportions, in (i) conventional jaws for tissue sealing or (ii)combination jaws for sealing-transecting instruments. In the embodimentdepicted in FIGS. 13–14, the structural body of the jaws 328 a and 328 bagain are preferably of an insulated material or separated from theelectrically-connected materials by an insulative layer.

A principal purpose for providing a flexible variably conductive matrix340 is to dynamically adjust the pressure of the engagement plane 325Aagainst the tissue volume that is compressed between the jaws. It isbelieved useful to provide a dynamic engagement plane since tissue mayshrink during a sealing procedure. The repose or untensioned shape ofthe variably conductive matrix 340 is shown as being convex, but alsocan be flat in cross-section or it can have a variety of differentgeometries or radii of curvature.

Of particular interest, the objective of a resilient jaw surface hasresulted in the development of an assembly of materials that can providea flexible and resilient engagement plane 325A at the surface of aresilient variably resistive material 340. In the embodiment of FIGS.13–14, it can be seen that the conductive portion or electrode 345 is athin metallic layer or member bonded to the variably resistive matrix340, which defines exposed surfaces 347 in the engagement plane 325A.The conductive electrode 325 again coupled to electrical source 150 andcontroller 155, as described previously.

Of particular interest, the variably resistive matrix 340 comprises asilicone material that can function as a PTC-type resistive matrix inthe same manner as the above-described ceramic materials. More inparticular, one embodiment of the variably resistive matrix 340 can befabricated from a medical grade silicone that is doped with a selectedvolume of conductive particles, e.g., carbon or graphite particles. Byweight, the ratio of carbon to silicone can range from about 90/10 toabout 30/70 to provide various selected switching ranges wherein theinventive composition then functions as a positive temperaturecoefficient (PTC) material. More preferably, the matrix is form about40% to 80% carbon with the balance being silicone. As describedpreviously, carbon types having single molecular bond are preferred. Onepreferred composition has been developed to provide a switching range ofabout 75° C. to 90° C. has about 50%–60% carbon with the balance beingsilicone. The variably resistive matrix 340 can have any suitablethickness dimension indicated at d₄, ranging from about 0.01″ to 0.25″depending on the cross-section of the jaws.

The electrode 345 that is exposed in engagement plane 325A can be asubstantially thin rigid metal, flexible foil, or a substantiallyflexible thin metallic coating. Such a thin flexible coating cancomprise any suitable thin-film deposition, such as gold, platinum,silver, palladium, tin, titanium, tantalum, copper or combinations oralloys of such metals, or varied layers of such materials. A preferredmanner of depositing a metallic coating on the polymer element comprisesan electrolysis plating process known in the art, such as provided byMicro Plating, Inc., 8110 Hawthorne Dr., Erie, Pa. 16509-4654. Thethickness of the metallic coating can range between about 0.0001″ to0.005″. Other similar electroplating or sputtering processes known inthe art can be used to create a thin film coating.

In operation, the working end of will function as described in the Types“A” and “B” embodiments. The elevation of the tissue temperature willconduct heat directly to the PTC matrix 340 and then will modulateenergy application between (i) active Rf heating, and (ii) passive orconductive heating. In addition, the resiliency of the PTC matrix 340will maintain substantially uniform pressure against the tissue even asthe tissue dehydrates or shrinks.

While the sectional view of the jaws 305A and 305B of FIGS. 13–14 depicta preferred embodiment, it should be appreciated that jaws 305A and 305Bcan use a compressible-resilient PTC matrix 340 in any of theelectrode-PTC matrix configurations shown in FIGS. 5–12, all of whichfall within the scope of the invention.

5. Type “D” Jaw Structure for Sealing Tissue.

An exemplary working end of a Type “D” probe 400 is shown in FIG. 15that again is adapted for energy delivery to an engaged tissue volumefor sealing or welding purposes. The jaws 405A and 405B defineengagement planes 425A and 425B as described previously that engagetissue from opposing sides. The lower (first) jaw 405A again carriesvariably resistive portion 440 that is a resilient sponge-type material,e.g., a silicone-based material, that is very similar to that describedin the Type “C” embodiment above. The PTC matrix 440 in this embodimentcomprises an open cell structure of the silicon polymer or other spongepolymer. More in particular, FIG. 15 illustrates working end 400 withlower (first) jaw 405A defining engagement plane 425A that contactstissue. The tissue-contacting plane of upper jaw (not shown) can be thesame as illustrated in FIG. 14 and comprise an insulator material.Alternatively, the upper jaw can carry electrically conductive bodycomponents that match the lower jaw. The jaw structure again is shown inFIG. 15 with a central channel 432 for accommodating a reciprocatablecutting member.

Of particular interest, in the embodiment of FIG. 15, the system isadapted to deliver saline flow from fluid source 460 directly throughthe open cell structure of the silicon-based PTC conductive layer 460.Such an open cell silicone can be provided adding foaming agents to thesilicone during its forming into the shape required for any particularworking end. The silicone has a conductive material added to matrix asdescribed above, such as carbon. In this embodiment, an exposedelectrode surface 445 comprises an elongate conductive element thatexposes portions of the compressible PTC conductive portion 440.Alternatively, the electrode surfaces can be a thin microporous metalliccoating, of the types described previously. The electrode 445 is shownas cooperating with a ground pad 458, although any of the electrode andresistive matrix arrangements of FIGS. 5–12 fall within the scope of theinvention.

In a method of using the jaw structure of FIG. 15, the system can applysaline solution through pores 462 in the open cell matrix 440 that areexposed in the engagement plane 425A that engages tissue. The method ofthe invention provides for the infusion of saline during an interval ofenergy application to engaged tissues to enhance both active Rf heatingand conductive heating as the jaws maintain tissue temperature at theselected switching range of the PTC matrix 440. In another aspect of theinvention, the compressibility of the silicone-based medial conductiveportion 440 can alter the volume and flow of saline within the open cellsilicone PTC portion 440. Since the saline is conductive, it functionsas a conductor within the cell voids of the medial resistive matrix 440,and plays the exact role as the carbon doping does within the walls ofcells that make up the silicone. Thus, the extent of expansion orcompression of the silicone medial conductive portion 440 alters itsresistivity, wherein the conductive doping of the material remainsstatic. It can be understood that a compression of PTC matrix 440 cancollapse the cells or pores 462 which in turn will restrict fluid flow.Thus, the system can be designed with (i) selected conductive doping ofsilicone PTC matrix 440 and (ii) selected conductivity of the salinesolution to optimize the temperature coefficient of the material underdifferent compressed and uncompressed conditions for any particulartissue sealing procedure. The sponge-type variably resistive bodyportion 440 can be designed to be a positive or negative temperaturecoefficient material (defined above) as the material expands to a reposeshape after being compressed. The resilient engagement surface 425A cannaturally expand to remain in substantial contact with the tissuesurface as the tissue is sealed and dehydrates and shrinks. At the sametime, the cell structure of the medial conductive portion 440 will tendto open to thereby increase fluid flow the engagement plane, which wouldbe desirable to maintain active and passive conductive heating of thetissue. Also at the same time, the selected temperature coefficient ofthe silicone PTC matrix 440 in combination with the saline volumetherein can insure that active Rf heating is modulated as exactlydescribed in the Types “A” and “B” embodiments above with any selectedswitching range. It is believed that the use of saline inflow will bemost useful in welding substantially thin tissue volumes that couldotherwise desiccate rapidly during active Rf energy delivery. Thus, thiseffect can be used to design into the working end certain PTCcharacteristics to cause the working end to perform in an optimalmanner.

It should be appreciated that the scope of the invention includes theuse of an open cell elastomer such as silicone to make both atemperature-sensitive variable resistive matrix and a form ofpressure-sensitive resistive matrix. As described above, the matrixunder compression will collapse pores in the matrix thereby making aconductively-doped elastomer more conductive. In effect, such a matrixcan be described as a pressure-sensitive matrix or a combinationtemperature-sensitive and pressure-sensitive variably resistive matrix.Further, an open cell elastomer that is not conductively-doped canfunction as a variably resistive matrix in combination with conductivefluid. When under compression, the conductive characteristics of thematrix would be lessened due to the outflow of the conductive fluids.Thus, it can be seen that the variably resistive matrix can be designedin a variety of manners to accomplish various Rf energy deliveryobjectives, all of which fall within the scope of the invention.

6. Type “E” Jaw Structure for Sealing Tissue

An exemplary working end of a Type “E” probe 500 is described withreference to FIG. 16. The Type “E” jaw of FIG. 16 is adapted forcontrolled energy delivery and is provided with additional features thatallows the variably resistive matrix to function optimally in workingends with a small cross-section. The objective to the Type “E”embodiment is to greatly reduce capacitive losses in operation.

In FIG. 16, it can be seen that working end 500 has first jaw 505A(second jaw 505B not shown) that extends along axis 507 with the lowerjaw alone carrying the active energy delivery components of theinvention, although both jaws could have such active components. Thelower jaw 505A defines an engagement plane 525 with cooperating jaw bodycomponents that comprise opposing polarity conductive portions 535A and535B with intermediate elements of a thermally-sensitive resistivematrix indicated at 540. For clarity of explanation, the opposingpolarity portions 535A and 535B are indicated as positive (+) andnegative (−). The resistive matrix 540 can be any of the types describedabove and preferably is a rigid ceramic-type positive temperaturecoefficient (PTC) material (see FIG. 3).

In the Types “A” and “B” working ends of FIGS. 5–9 above, the active jawcarried opposing polarity conductive portions and with an intermediatelayer of a variably resistive matrix-which is similar to the Type “E”working end 500. As can be seen, for example in FIG. 6, the transversedimension td across the variably resistive matrix between the opposingpolarity electrodes can be substantially small due to the thin layers ofmaterial. The direction of current flow is indicated at A in FIG. 6which is generally transverse to the axis 107 of the jaws (see FIG. 6).

The Type “E” embodiment of FIG. 16 provides an improved manner ofarranging the conductive components of a working end to reducecapacitive coupling of opposing polarity conductive portions 535A and535B across the variably resistive matrix 540. More in particular, theType “E” positions elements of the opposing polarity conductive portions535A and 535B is such a manner to induce a selected directional currentflow through the resistive matrix 540, with the preferred directionbeing any elongated dimension within the jaw structure—typically not atransverse direction across the jaw. The typical preferred direction forinducing current flow is an axial direction in relation to axis 507,with such current flow through matrix indicated at arrow AA in FIG. 16.It should be appreciated that the arrangement of jaw components in FIG.16 can be provided in jaws that have a channel for receiving areciprocating blade member as shown in FIGS. 13–14, or combined with anyof the electrode configurations of FIGS. 5–12.

In one Type “E” embodiment corresponding to the invention is depicted inFIG. 16, the jaw has at least one volume of resistive matrix 540 thatcomprises an interior body portion of the jaw (similar to the Type “A”embodiment of FIG. 2). Each said volume of resistive matrix 540 hasfirst and second ends 542 a and 542 b that contact the respectiveopposing polarity conductive portions 535A and 535B. The axial dimensionad between the first and second ends 542 a and 542 b can be any suitabledimension and can be substantially greater than the thickness dimensiontd of the resistive matrix 540. The conductive portions 535A and 535Beach have a projecting leg portion (546 a and 546 b) that contact thefirst and second ends 542 a and 542 b of the resistive matrix 540. Ascan be seen in FIG. 16, the body portions of the resistive matrix 540,except for the first and second ends 542 a–542 b thereof, are surroundedby electrically insulative layers indicated at 548. The opposingpolarity conductive portions 535A and 535B are coupled to a voltagesource. Thus, it can be understood how current is induced to flow in thedirection of arrow AA through the matrix 540 to reduce capacitivecoupling across the resistive matrix 540. The insulative layer 548 canbe any type of material or layer, for example, a thin layer of atitanium oxide ceramic-type material. In another embodiment (not shown),the insulative layer 548 can be an air space. In yet another embodiment,the insulative layer 548 can any combination of air spaces and any otherinsulative material.

7. Type “F” Jaw Structure for Sealing Tissue

FIG. 17 illustrates one jaw of a Type “F” system 600 corresponding tothe invention. The jaw structure again is adapted for controlled energydelivery to tissue utilizing a variably resistive matrix 640. The lowerjaw 605A defines an engagement plane or surface 625 for contactingtissue that overlies two variably resistive body portions indicated at630 and 640. The core of the jaw 605A again carries a conductive bodyportion indicated at 645 that is coupled to a voltage source asdescribed previously. The interior variably resistive matrix 640 is athermally sensitive material as described in the Types “A” and “B”embodiments, for example, a PTC matrix of a ceramic material. Anoptional structural body of the jaws is indicated at 647 which isinsulated from the above-described electrically active components.

Of particular interest, the exterior variably resistive matrix 630 is ofa pressure-sensitive resistive material that is carried across theengagement plane 625. In one embodiment, such a variably resistive layer630 can be substantially thin and fabricated of a material described asa “pressure variable resistor ink” and is more specifically identifiedas Product No. CMI 118-44 available from Creative Materials Inc., 141Middlesex Rd., Tyngsboro, Mass. 01879. The resistance vs. pressurecharacteristics of the pressure-sensitive resistive matrix 630 can beadjusted by blending the above-described material with Product No. CMI117-34 that is available from the same source.

In operation, it can be understood that any pressure against thepressure-sensitive resistive layer 630 will locally decrease itsresistance to current flow therethrough. As the jaws are closed, theengagement plane 625 of the lower jaw 605A will be pressed against thetissue. Due to the potential pressure vs. resistivity characteristics ofthe resistive layer 630, the layer can be designed so that Rf currentwill only flow through localized portions of the engagement plane 625where the pressure-sensitive resistive layer 830 is under substantialpressure, which in turn locally lowers the resistance of a portion ofthe surface layer. Further, the interior thermally-sensitive variablesensitive resistive layer 640 will modulate Rf flow as previouslydescribed to maintain a targeted tissue temperature.

It should be appreciated that the scope of the invention and its methodof use of the includes the use of jaw working surface similar to that ofFIG. 17 that does not carry an interior body portion of thethermally-sensitive resistive matrix indicated at 640. In other words,the jaw can rely only on the pressure-sensitive resistive layer 630about the engagement plane 625 to locally apply energy to capturestissue volumes.

In another embodiment (not shown), either one both jaws can have anelongate core of the substantially resistive material in addition to thecore electrode and a variably resistive matrix of any type describedabove. The resistive material has a fixed resistance and is adapted topre-heat the jaw, its engagement plane and the engaged tissue as a meansof pre-conditioning the tissue to attain a certain selected impedance.Such a system will be useful when the engagement plane is large indimension. A thermally conductive, but electrically insulative, layercan be disposed intermediate the fixed resistance material and aconductive (electrode) layer. The conductive layer is coupled in serieswith the fixed resistance material to the remote voltage source. Thevariably resistive matrix is disposed between the engagement plane andthe conductive (electrode) layer—as described in any of the Types “A”and “B” embodiments above.

Those skilled in the art will appreciate that the exemplary systems,combinations and descriptions are merely illustrative of the inventionas a whole, and that variations of components, dimensions, andcompositions described above may be made within the spirit and scope ofthe invention. Specific characteristics and features of the inventionand its method are described in relation to some figures and not inothers, and this is for convenience only. While the principles of theinvention have been made clear in the exemplary descriptions andcombinations, it will be obvious to those skilled in the art thatmodifications may be utilized in the practice of the invention, andotherwise, which are particularly adapted to specific environments andoperative requirements without departing from the principles of theinvention. The appended claims are intended to cover and embrace any andall such modifications, with the limits only of the true purview, spiritand scope of the invention.

1. A working end of a surgical instrument for delivering energy totissue, comprising: paired first and second jaw members moveable betweenan open position and a closed position; and at least one jaw includingan electrode, a first body portion comprising an electrically conductivematerial having a temperature-sensitive variable resistance whichincreases with an increase in the temperature of the material forresponding to the temperature of the tissue, and a second body portioncomprising a conductive material, the second portion configured to becoupled to an Rf voltage source, the electrode and the second bodyportion configured to be coupled to separate poles of the Rf voltagesource, wherein the first body portion is disposed between the secondbody portion and the electrode to control the passage of electricalcurrent from the second body portion to the electrode and wherein saidat least one jaw defines a surface engagement plane for engaging tissue,said engagement plane carrying an exposed surface of said first bodyportion.
 2. The working end of claim 1, said engagement plane carryingan exposed surface of said second body portion.
 3. The working end ofclaim 1, wherein said first body portion comprises a ceramiccomposition.
 4. The working end of claim 1, wherein said first bodyportion defines a switching range at which its electrical resistancesubstantially varies in a selected temperature range.
 5. The working endof claim 4, wherein said switching range falls between about 40° C. and200° C.
 6. A working end of a surgical instrument for delivering energyto tissue, comprising: paired first and second jaw members moveablebetween an open position and a closed position; and at least one jawincluding an electrode adapted for contacting tissue, a first bodyportion comprising an electrically conductive material having atemperature-sensitive variable resistance which increases with anincrease in the temperature of the material for responding to thetemperature of the tissue, and a second body portion comprising aconductive material, the second portion configured to be coupled to anRf voltage source, the electrode and the second body portion configuredto be coupled to separate poles of the Rf voltage source, wherein thefirst body portion is disposed between the second body portion and toelectrode to control the passage of electrical current from to secondbody portion to the electrode responsive to the temperature of contactedtissue and wherein said at least one jaw defines a surface engagementplane for engaging tissue, said engagement plane carrying an exposedsurface of said first body portion.
 7. The working end of claim 6, saidengagement plane carrying an exposed surface of said second bodyportion.
 8. The working end of claim 7, wherein said first body portioncomprises a ceramic composition.
 9. The working end of claim 6, whereinsaid first body portion defines a switching range at which itselectrical resistance substantially increases or decreases in a selectedtemperature range.
 10. The working end of claim 9, wherein saidswitching range falls between about 40° C. and 200° C.